The use of stent to keep a blood vessel or other body lumen open in a human body has become a very effective therapy to treat blood vessel stenosis and lumen obstruction. For example, using stent to treat coronary and peripheral artery blockage has become a common practice. Stents have been successfully used in keeping passageways open such as the prostate, urethral, the esophagus, the biliary tract, and intestines.
There are two types of stents that are widely used and/or studied nowadays: metallic stents and bioabsorbable polymer stents. Currently, most of the stents clinically available are metallic stents. However, there are some disadvantages that are associated with the use of metal stents. Because the metals are typically much harder and stiffer than the surrounding lumen tissue, metal stents may result in an anatomical or physiological mismatch, thereby cause irritation to the surrounding lumen tissue. The metal struts of the stents, once break after implantation, may pierce through the body lumen and causing some serious complications. The permanent irritation caused by the presence of this metallic stent will eventually cause lumen area loss due to the overgrowth of the irritated tissue. Further, metallic stents prevent lumen expansion associated with late favorable remodeling. Metal stents impair the vessel geometry and often jail and obstruct side branches.
In contrast, stents made from polymers are less stiffer than metals. Therefore, will resolve most of the above issues that are associated with the metal stents. One type of polymer stents is bioabsorbable stent. Bioabsorbable polymer stents, once they are bioabsorbed, leave behind only the healed natural vessel, allowing restoration of vasoreactivity with the potential of vessel remodeling. Late stent thrombosis is unlikely since the stent is gone, and prolonged anti-platelet therapy is not necessary in this instance.
Polymer stents can also be suitable for complex anatomy where currently used metal stents impede on vessel geometry and morphology and are prone to crush and fractures, such as in saphenous femoral and tibial arteries. Bioabsorbable stents can be used as a delivery device for agents such as drugs and genes, and may be used for treatment of vulnerable plaque.
Flexibility of the stents is one of the important characteristics. Stents have to be flexible in their crimped state in order to facilitate the delivery of the stent, for example within an artery. In some cases, stents also have to be flexible after being deployed and expanded, especially when a stent may be subjected to substantial flexing or bending, axial compressions and repeated displacements at points along its length, for example, when stenting the superficial femoral artery. This can produce severe strain and fatigue, resulting in failure of the stent.
Typically, the main body of a stent is made from a single type of material, such as stainless steel, nitinol, Co—Cr alloy, polyL-lactide. Fabrication methods, such as laser cut, braiding, and thermal forming are often been used. The mechanical properties of the stents can only be changed by the change of the structure design of the stents once the material has been chosen.
One primary goals of stent designs has been to insure that the stents have sufficient radial strength so that, when it is delivered to the intended treatment location and expanded, they can sufficiently support the lumen. Stents with high radial strength, however, tend also to have a higher longitudinal rigidity or less flexible than the vessels which are implanted. When a stent has a higher longitudinal rigidity than the vessel in which it is being treated, there is a higher chance that the rigid stent will cause trauma to the vessel at the ends of the stent, due to stress concentrations caused by the mismatch in compliance between the stented and un-stented sections of the vessel. Furthermore, for a stent with higher longitudinal rigidity, after deployment in certain applications it may be subjected to substantial flexing or bending, axial compressions and repeated displacements at points along its length, for example, when stenting the superficial femoral artery. This can produce severe strain and fatigue, resulting in failure of the stent.
In a stent that is made from a single material, the mechanical properties of the stents and biological performance are largely determined by stent's structure design. It is conceivable that the mechanical and biological prosperities may be further modified if a structure component from a different type of material can be incorporated into the main structure of the stent.
Stents fabricated using a laser cut method are cut from a tube which is produced from a single material. Thus, a different structure component of a different polymer cannot be incorporated into the main stent structure during this laser cutting process. The different polymer component can only be attached to the stent main structure after obtaining the main stent structure. U.S. Patent application No. 2009 0,234,433 entitled “Helical Hybrid Stent” disclosed both a single and multi-helical structure aimed at improving the longitudinal flexibility. In this patent application, the inventors used a polymer component, such as a polymer fiber layer, attached to the outer-surface to maintain the tubular shape of the stent so that the main stent component is able to provide structural support both to the vessel and the polymer fiber layer upon deployment. In other words, polymer component is not a integrated part of the main stent structure.
We previously have filed a U.S. patent application (Liu, et al, U.S. patent application No. 2010/0330144) in which a rapid polymer stent fabrication method and system was disclosed. Unlike a laser cut stent fabrication process, this polymer stent fabrication system uses thermal plastic polymer pellets and powders to directly produce polymer stents in a single step. We further modify the system so that two or more polymers can be used and extruded in a sequential way to produce a polymer stent with two or more polymer structure component within its main body structure.